Transportation, storage and/or distribution of industrial gases such as oxygen, nitrogen, helium, hydrogen, argon, neon and the like with atmospheric pressure condensation temperatures in the cryogenic range, e.g. below -130.degree. F. (-90.degree. C.) becomes most economical when the gases are maintained in the liquid phase. In the ordinary course of liquefying the gases, large quantities of refrigeration are necessary. When the liquid is brought to a user's location and placed in a cryogenic storage dewar, the liquid is generally converted to a gas for use by the customer thus releasing refrigeration. Conventional storage dewars dump the refrigeration produced by vaporation of the stored cryogen to the atmosphere.
In the case of long term storage of cryogen on a customer site there is the possibility, if the withdrawal rate is not very large, that due to infiltration of heat, stored cryogen will be vaporized inside the dewar and the vapor vented to the atmosphere, thus causing loss of cryogen available for use by the user (customer). For example, when liquid hydrogen is stored in a conventional 1500 gallon cryogenic dewar comprising an inner vessel and an outer vessel with a vacuum space between the inner and outer vessel and the vacuum space containing a conventional multi-layer insulation, in order to prevent loss of stored cryogen by venting, the user must draw off a minimum of approximately 80,000 ft.sup.3 per month of vapor in order to utilize all of the delivered cryogen.
One approach to maintaining liquid inventory and prevent venting of vaporized cryogen inside the tank is disclosed and claimed in U.S. Pat. No. 3,698,200. Patentees' invention relies upon a heavy radiation shield disposed between the inner and outer vessel which is cooled by the product as the inner vessel is filled. This technique is used in combination with the conventional multi-layer insulation, vacuum jacketing, and a liquid nitrogen reservoir inside the tank and is particularly adapted for storage of liquefied helium which vaporizes at approximately 4.degree. K.
Precooling of the radiation shield according to Patentees is accomplished by allowing a portion of the fluid stored inside the tank to vent during filling to thus precool the radiation shield.